1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the accuracy of a thread groove and to a method for evaluating the accuracy, and in particular, to an improvement in the method and the apparatus in which the accuracy of a thread groove in a depth direction and in a thread axis line direction can be measured by a unidirectional (one-dimensional) detection device, and at the same time, not only the local accuracy of the thread groove but also the overall accuracy can be easily grasped.
2. Description of the Relevant Art
A prior art apparatus is disclosed, for example, in Japanese Patent Laid-Open Publication No. 54-130156 entitled "Automatic Measuring Machine of Thread Elements".
This prior art relates to a measuring machine for measuring elements of a thread such as a pitch, a taper, an effective diameter, an outer diameter and the like, and in this machine, a measuring head includes a minute displacement detector capable of detecting two-dimensional displacement quantity and a contactor connected to the minute displacement detector and in contact with both opposing flanks of a thread groove, and the measuring head is used to measure displacements in a radial direction and in an axial direction over the whole length of the thread groove. Then, each element of the thread is determined on the basis of the results of the measurement. Furthermore, the outer diameter of the thread can be obtained by bringing the contactor of the measuring head into contact with the top of the thread, and by performing a similar measurement.
However, in the prior art machine, since the two-dimensional (in a depth direction and a width direction of a thread groove) displacements have to be detected, a large and expensive two-dimensional minute displacement detector is required. Moreover, in the case of measuring a thread groove, since the contactor must be always in contact with both flanks in order to measure accurately, a high measuring pressure must be applied to the measuring head, and thus, the machine becomes large in size, and as a result, it is not suitable to measure a female thread in which available space is narrow.
Furthermore, in measuring the accuracy of a thread groove, in the case of a nut having a ball thread and a screw shaft, the accuracy of loci of contact points of a ball used therein with both flanks is important, and in the case of outer threads, the accuracy of a position of an effective diameter is important. Accordingly, in order to perform an accurate measurement, it is necessary to prepare and replace with a contactor having a radius size corresponding to a size of the thread. As a result, it is cumbersome, the number of parts is increased, and the apparatus becomes expensive.
Further, a contactor having a predetermined radius size must be used, and this radius size is much larger than minute irregularities of a surface to be measured. Therefore, it is impossible to measure a minute shape of the surface to be measured.
In addition, in the prior art technique, although locally accurate measured data of a thread groove is easily obtained, no method for easily recognizing the overall accuracy is available, and thus, it would further be desirable to develop a method for feeding back the measured data effectively to a manufacturing process.